Nitrogen Use and Uptake in Corn
Understanding nitrogen (N) uptake patterns is integral to improvement of corn nitrogen use efficiency. It allows growers to plan, manage and provide optimal N supplies to the crop that support maximize yield potentials. Nitrogen (N) remains the most commonly applied nutrient and one of the costliest inputs in corn production. N application averages 18% and 13% of the variable costs in a corn-corn and corn-soybean rotation, respectively (Duffy 2014). The cost-to-benefit ratio usually exceeds that of other fertilizer inputs, but growers have many options to improve N use efficiency to maximize its value. These include minimizing N losses and applying optimal rates at ideal times that coincide with peak uptake by the crop.
NITROGEN UPTAKE IN CORN
Timing of N uptake and sources of N utilization by corn include:
- N for grain development originates from both remobilized N from vegetative tissues and continued N uptake from the soil. Therefore, ensuring a season-long N supply is critical for maximizing yield.
- By flowering (R1), corn has taken up approximately 63% of its N requirement for the season. The rest is taken up during the grain-fill period (R1 to R6).
- With high yields, ~140 to 210 lbs N/acre is needed to support grain development. Approximately 38% of this demand is remobilized from vegetative tissue; the rest is supplied from continued uptake after flowering.
- In high-yield environments, post-flowering N uptake can range from 85 to 130 lbs N/acre.
Figure 1. Seasonal N uptake (lbs N/acre) separated into leaf, stalk, tassel, husk+cob, and grain material for a typical corn crop. Abendroth et al., Iowa State University Extension, 2011.
N applied closer to maximum crop use is less likely to be lost and more likely to be taken up by the crop and potentially available to support kernel set at flowering and late-season grain development.
In-season soil N monitoring could be a helpful tool to alert growers about potential N shortfalls so action plans could be considered, and guide N management plan modifications for future production years.
The plant’s N demand for grain development is roughly proportional to the grain yield. A review of the most recent literature for modern corn hybrids indicates that at yields between 80 and 150 bu/acre, the demand is less than 65 lbs N/acre). When yields exceed 200 bu/acre, grain N removal averages 139 lbs N/acre. The Iowa State University reference guide, Corn Growth and Development, reports an almost identical amount of N in the grain - 137 lb N/acre at yield levels of 225 bu/acre (Figure 1).
TIMING AND SOURCE OF N UPTAKE
The importance of N uptake prior to flowering cannot be overstated, as this N supports critical ear shoot development, kernel number and potential kernel size.
For this reason, corn growers have traditionally targeted N availability to this period and considered post flowering applications to be of little value. Historical research with older, lower-yielding hybrids also pointed to early application.
In fact, many studies concluded that grain fill depended almost entirely on remobilized N from the leaves and stalk, and that post flowering N uptake contributed little to yield accumulation.
However, extensive research studies conducted by numerous university and independent scientists over the last 5 years have shown that nitrogen needed for grain development originates from both remobilized N (from leaves, stalks, cobs and husks) and continued N uptake from the soil.
A study conducted at Macomb, Ill., in 2012, compared a “normal” rate of 200 lb N/acre with a “low” rate of 50 lb N/acre. Resulting yields averaged 250 bu/acre for the normal rate, and 100 bu/acre for the low N rate. In the normal N environment, 130, 170, and 301 lbs N/acre were taken up by V12, R1 and R6, respectively (Figure 2). Of the 195 lb N/acre contained in the grain at maturity, 63 lbs were sourced from remobilized N (from leaves, stalks, etc.) and 132 lbs were from N taken up post-flowering (Figure 2).
Figure 2. Seasonal N uptake (lbs N/acre) for hybrids grown under normal (left) and low (right) N supply near Sciota, Ill., 2011.
In the low N environment (100 bu/acre yield level), N uptake was limited to 60 lbs/acre by R1, and only 27 lbs/acre were taken up post flowering (Figure 2). These reduced N levels simulate conditions when the pool of available N is low due to leaching, denitrification or under-application of N.
In cases of low N availability when post flowering N uptake cannot fully support grain development, the remaining nitrogen comes from N remobilized from the vegetative tissues (stalk, leaves, husks and cob). The leaves are the most substantial source of remobilized N. Based on recent publications from experiments conducted in Illinois and Indiana, the amount of N remobilized from vegetative tissue averages 38% across all yield levels with a maximum of 54% under high yield, high N conditions. In a recent publication by DeBruin et al., (2012) 2 independent studies documented that a maximum of 63% of the leaf N could be remobilized to the grain. Further work by Pioneer documented that the stalk generally contributes less than 20% of the remobilized N to the grain and that contributions from the cob and husks are insignificant.
Several university studies evaluated timing of N uptake. In Iowa State studies, approximately 60% (120 lbs N/acre) of total N (Figure 1) was taken up and stored in the leaves, stalk and ear shoot by R1 (silking) for a high-yielding corn crop of 225 bu/acre corn (Abendroth et al., 2011). By R6 (black layer), total N uptake (stover + grain) reached 190 lbs N/acre. Further calculations with these data indicate that 70 lbs of N/acre must still be taken up post-flowering to support grain development.
Figure 3. Percent of N taken up by the plant before and after flowering, and percent of N in the grain from post-flowering (after VT-R1) uptake and remobilized sources.
Other recent publications document the amount of N taken up by flowering and after flowering (Figure 3). These data suggest that the total N taken up by flowering is almost identical under low or normal N conditions (averaging 63% and 62%, respectively). While this is a large proportion of total seasonal N, the demand for post-flowering N uptake still ranges from 39 to 132 lbs N/acre.
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